This invention relates to telephone systems. The invention has been developed primarily in relation to the requirements which arise in respect of a telephone system in which certain of the stations to and from which calls require to be made are situated in a hazardous area, that is to say an area in which the gaseous environment is of a composition such that there is a risk of ignition, as, for example, occurs in underground mine workings.
It will, however, be understood that the invention is of general application in environments in which like or analogous requirements arise.
In underground mine workings, and more especially those in which mining operations are conducted by what is known as long wall mining method, the coal or other mineral to be extracted is removed from a long face. At one end there is an inlet tunnel or roadway leading up to the face along which incoming supplies are brought and at a position spaced along the coal face, an outlet roadway or tunnel leads away from the coal face, the mined coal or mineral material normally being conveyed away along the outlet roadway.
The area which extends along the face and for some distance along both inlet roadway and outlet roadway and along a main roadway leading, possibly as far as the bottom of the mine shaft, is usually designated a hazardous area, and it is necessary that all electrical equipment installed and operating in such area shall comply with a safe specification. The regulations made by Governmental agencies vary from one country to another but it is convenient to refer to those which govern the use of electrical equipment in underground mine workings in Great Britain where, generally speaking, electrical equipment has to comply with one of two requirements, namely either that it shall be in accordance with an "intrinsically safe specification" or shall be contained within a flameproof casing, the term "flameproof" being defined in more detail in British Standards relative to the particular equipment and conditions under which it is used.
Intrinsically safe specifications in Great Britain are determined upon the basis of current and voltage values across a spark gap which will not produce ignition in the atmosphere under consideration. As a guide to designers, the current and voltage values have been made available by way of a graph containing a family of curves, one for each atmosphere, with voltage values plotted as abssissae and current values plotted as ordinates. The general form of such curves is that each slopes downwardly from a point near the Y-axis at a decreasing gradient (negative), approaching X-axis more or less asymptotically. A designer can thus select an appropriate voltage (or current) dependent upon the nature of the circuit to be designed, and from the curve applicable to the particular atmosphere for which the circuit is required to be intrinsically safe, the designer can then select the current (or voltage). The circuit requires to be designed so that under the worst fault conditions (for example a short circuit), this current will not be exceeded. Particular circuit configurations still require to be passed as intrinsically safe by the authorities since some departure from the curves of the guidance graph may be required (or permitted) because of variation in circuit parameters (more particularly inductive impedance presented at the possible short circuit point).
The curves of the guidance graph provided by the authority pursuant to British Standard BS1259 of 1958 are classified as pertaining to the operation in the following atmospheres:
Class 1 -- methane
Class 2 -- pentane
Class 2d -- ethylene
Class 2e -- hydrogen.
A further factor which affects the readiness with which the gas may become ignited is the particular identity of the metal present in the conductors and components of the electrical circuit intended for use in the hazardous areas.
Certain metals which are commonly used in electrical conductors and components do increase the possibility of ignition of the gas concerned, and their presence then demands a higher standard of the intrinsically safe specification than would be the case where in a given atmosphere were such metal absent.
One of the metals concerned is cadmium which if present at the boundaries of the spark gap, whether produced by breakage or disconnection of the conductor or otherwise, tends to prolong and increase temperature of the spark produced.
In the art there has thus arisen the use of an expression "cadmium safe", as qualifying an intrinsically safe circuit. This means not that the circuit necessarily contains cadmium, but that it complies with a higher standard of intrinsically safe specification for use in a given atmosphere than would be called for upon the basis of that atmosphere alone.
Reverting to the problem of telephone systems in hazardous areas, and in particular in underground mine workings, one of the basic problems which is encountered is that, in order to make an outgoing call, that is to say from some site in the hazardous area by means of a telephone unit situated thereat, to a remote station, such, for example, as a telephone exchange on the surface, it is necessary that the D.C. loop be completed in the exchange in order to produce response of the exchange to an incoming call signal from the telephone unit. Such call signal normally consists of an initial signal (which is simple completion of the D.C. loop) followed by a series of D.C. pulses produced by operation of a dialling means provided at the telephone unit. Further, in order to make an incoming call, some form of call indicating device has to be energised to alert the user to the incidence of the incoming call. In a conventional telephone unit such indicating device is in the form of an electrically operated bell.
If the exchange were connected by a simple transmission line extending down the mine shaft and along the main roadway as necessary, to a site in the inlet roadway or at the coal face, the current value which would be established in conductors of the transmission line to the telephone unit, if conventionally constructed, during initiation of either an outgoing or incoming call would exceed the intrinsically safe value of current. For outgoing calls the current in the D.C. loop would exceed the intrinsically safe value, while for incoming calls the "ringing tone" current for operating the call indicating device would exceed the intrinsically safe value.